Driving mechanism for slide changers or the like

ABSTRACT

The slide changer and the indexing mechanism for the tray of a slide projector are operated intermittently by a power train which is connected to the output shaft of an electric motor. An electromagnet is energized in response to opening of the motor circuit to bring about rapid stoppage of the slide changer and indexing mechanism in a predetermined position. Energization of the electromagnet results in frictional engagement of an element of the power train with a movable or stationary part of the electromagnet. A spring is provided to disengage the element of the power train from the stationary or movable part in response to deenergization of the electromagnet.

United States Patent Inventor Josef Scheibel Ober-Morlen, Germany Appl. No. 855,529 Filed Sept. 5, 1969 Patented May 25, 1971 Assignee Braun Aktiengesellschaft Frankfurt, Main, Germany Priority Oct. 2, 1968 Germany P 18 00 516.8

DRIVING MECHANISM FOR SLIDE CHANGERS OR THE LIKE Primary ExaminerLeonard I-I. Gerin Attorney-Michael S. Striker ABSTRACT: The slide changer and the indexing mechanism for the tray of a slide projector are operated intermittently by a power train which is connected to the output shaft of an electric motor. An electromagnet is energized in response to opening of the motor circuit to bring about rapid stoppage of the slide changer and indexing mechanism in a predetermined position. Energization of the electromagnet results in frictional engagement of an element of the power train with a movable or stationary part of the electromagnet. A spring is provided to disengage the element of the power train from the stationary or movable part in response to deenergization of the electromagnet.

PATENTEUHAY2S|97| 3580.096

SHEEI 1 [IF 3 INVENTOR oser SKHLI/GEG ATTORNEY PATENTEU HAYZSIHTI 3580.096:

INVENTOR me; scusran OM/ u a-bu ATTORNEY DRIVING MECHANISM FOR SLIDE CIIANGERS OR THE LIKE BACKGROUND OF THE INVENTION The present invention relates to driving mechanisms in general, and more particularly to improvements in driving mechanisms which can arrest one or more rotary, reciprocable or otherwise movable parts in accurately determined positions. Still more particularly, the invention relates to improvements in mechanisms which can be utilized in slide projectors to impart motion to one or more driven parts and to arrest such parts or parts in one or more predetermined positions.

It is already known to drive the slide changer and/or to index the tray of a slide projector by means of a mechanism which includes an electric motor or another suitable prime mover and power train which is normally a gear train or a link train. As a rule, such mechanism must arrest the driven part or parts in accurately determined positions, especially when the driven parts include a mask or shield which blanks the projec- ,tion screen during transport of slides between the tray and a position of registry with the projection lens. Problems arise when the prime mover is an electric motor which has a tendency to continue to run after the motor circuit is opened in response to opening of a switch or the like. It was found that the driven parts continue to move subsequent to opening of the motor circuit even if the power train includes a stepdown transmission with a large number of intermeshing gears. The inertia of moving parts causes the driven element or elements to perform at least some further movements after the motor circuit opens.

Attempts to effect rapid stoppage of driving mechanisms whose prime mover is a DC motor include the'provision of a resistor which is connected in parallel with the armature of the motor. Such proposals have met with limited success, especially in connection with high-speed electric motors. It was found that the driven elements which derive motion from such motors continue to move after the motor circuit is opened whereby the customary limit switches which are installed in the motor circuit are likely to again complete the motor circuit before the driven elements are brought to a full stop. Such limit switches are provided to determine the stages of a cycle during which the 'tray of a slide projector is indexed by a step and/or during which the slide changer begins to move a slide from the tray toward the projection position or vice versa.

SUMMARY OF THE INVENTION An object of the present invention is to provide a compact, simple and reliable driving mechanism which can move one or more driven elements between accurately detennined positions.

Another object of the invention is to provide a driving mechanism whose prime mover preferably includes an electric motor and which is especially suited to effect movements of the tray, slide changer, mask, projection lens and/or other driven elements in a slide projector.

A further object of the invention is to provide the aboveoutlined driving mechanism with a novel braking device which can arrest one or more driven parts in accurately determined positions even if the prime mover tends and actually continues to run subsequent to opening of its circuit.

An additional object of the invention is to provide a novel driving mechanism of the type wherein the motor is preferably operated intermittently to thereby actuate the slide changer, mask, tray and/or other movable parts of a slide projector or the like.

The improved driving mechanism is particularly suited for use in slide projectors to effect intermittent movements of slide changers, masks, trays, projection lenses and/or other parts. It comprises a prime mover, preferably an electric motor having an output member, a driven element (e.g., a camshaft which can transmit motion to a slide changer and/or to the indexing mechanism for a slide tray), a power train for connecting the output member of the prime mover with the driven element and including a plurality of motion-transmitting elements, braking means including one or more electromagnet means energizable to thereby exert a braking ac-- tion upon at least one of the elements (i.e., upon a motiontransmitting element and/or upon the driven element), and programming or regulating means (preferably including one or more electric switches) for energizing the electromagnet means in predetermined positions of the driven element. Such switch or switches may be actuated by one or more cams provided on a camshaft which constitutes or transmits motion to a driven element.

The electromagnet means may include a stationary portion which attracts the one element when the electromagnet means is energized, i.e.,.the one element then constitutes the movable armature of the electromagnet means. Altemately, the electromagnet means may include a movable armaturewhich is moved into friction-generating engagement with the one element in response to energization of the electromagnet means. If the one elementforms part of the power train, the latter may include a coupling which is automatically disengaged on energization of the electromagnet means to thereby interrupt the power train so that the prime mover can run itself out without transmitting motion to the driven element'and that the latter can be more readily braked by the electromagnet means.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved driving mechanism itself, however, both as to its construction and its mode of operation, together with additional featuresand advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of a driving mechanism which embodies one form of the invention and is used to transmit motion'to the slide changer and tray of a slide projector;

FIG. 2 is a fragmentary schematic plan view of a portion of a slide projector and illustrates the manner in which the driven element of the mechanism shown in FIG. 1 cooperates with the slide changer and with the indexing mechanism for the tray;

FIG. 3 illustrates the electric circuit of the driving mechanism shown in FIG. 1;

FIG. 4 is a fragmentary plan view of a second driving mechanism, showing the electromagnet of the braking device in deenergized condition;

FIG. 5 illustrates the structure of FIG. 4, but showing the electromagnet in energized condition; and

FIG. 6 is a fragmentary partially plan and partially sectional view of a third driving mechanism.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drive mechanism of FIG. 1 comprises a prime mover 1, here shown as an electric motor, and a power train 2. This power train is a gear train which includes a series of motiontransmitting elements including a pinion 4 on the rotary output shaft 3 of the motor 1, a gear 4a which meshes with the pinion4, a pinion 4b which is coaxially secured to the gear 4a, a gear 7 which meshes with the pinion 4b, a pinion 7a which is coaxially secured to the gear 7, a gear 7b which meshes with the pinion 7a, a pinion 7c coaxially secured to the gear 7b, and a gear 5 which drives a camshaft 6. The camshaft 6 is the driven element of the mechanism and serves to transmit motion to the slide changer (not shown) and to a transmission which indexes the slide tray or magazine (not shown).

The gear 7 of the power train 2 is provided with a concentric ring 8 of soft iron. This ring 8 is applied to one end face of the gear 7 and is adjacent to a leaf spring 9 which is aflixed to a bracket 10 or an analogous carrier installed in or forming part of the slide projector housing. The leaf spring 9 serves as a support for an electromagnet 11 having a core 12 consisting of iron. When the electromagnet 11 is deenergized, the forward end of the core 12 is separated from the ring 8-bya narrow airgap. When energized the electromagnet 11 moves toward and engages the ring 8 by simultaneously stressing the leaf spring 9. The resulting friction between the core 12 of the electromagnet 11 and the ring 8 gradually brakes the gear 7 andhence the power train 2 to arrest the camshaft 6 in a desired angular position. A ferrotype yoke 12a of the electromagnet 1.1 can engage the gear 7 in response to energization of the electromagnet 11 to produce additional friction and to thus enhance the braking action.

FIG. 2 shows that the camshaft 6 constitutes the input shaft of a transmission 15 which transmits motion to a slide changer l6 and to an indexing mechanism 17 for the slide tray. The camshaft 6 is provided with two axially spaced actuating cams or trips 18, 19 having lobes disposed diametrically opposite each other with reference to the axis of the shaft 6. The cam 18 serves as a means for actuating a first two-way electric limit switch A having a movable contact and two fixed contacts 20a, 20b. The other cam 19 serves as an actuating means for a second two-way electric limit switch B having a movable contact 21 and two fixed contacts 21a, 21b. The limit switches A and B start and terminate various movements of the parts which effect movements of slides between the tray and a projection position (of registry with the projection gate and objective). The cams 18, 19 determine the sequence in which the various movements begin and terminate in response to rotation of the camshaft 6. As shown in FIG. 3, the limit switches A and B are in circuit with the motor 1, with a source 24 of electrical energy, and with the electromagnet 11. FIG. 3 further shows a manually operated control switch or selector switch D having a movable contact 22 and two fixed contacts 22a, 22b, and a master switch E. which is a two-position switch and includes a movable contact 23 which can assume the solid-line position 23a and the phantom-line position 23b.

The operation:

The selector switch D serves to select either one of two modes of operation of the slide projector. One mode of operation involves a full slide change which takes place in response to rotation of the camshaft 6 through 360. During such full revolution of the camshaft 6, the slide changer 16 moves a slide from projection position back into the corresponding compartment of the tray, the mechanism 17 indexes the tray to move a fresh slide into registry with the slide changer l6, and the latter thereupon moves the fresh slide into the projection gate. The other mode of operation involves rotation of the camshaft 6 through 180. The slide changer 16 thereby moves a slide from projection position back into the corresponding compartment of the tray and the tray is ready for indexing to place a selected slide into the range of the slide changer.

FIG. 3 illustrates the selector switch D in that position in which the movable contact 22 engages the fixed contact 220. The slide projector is then set to effect rotation of the camshaft 6 through a full revolution in response to short-lasting depression of the movable contact 23 in the master switch E. A slide is assumed to be located in the projection position; therefore, the movable contact 20 of the limit switch A engages the fixed contact 200 and the movable contact 21 of the limit switch B engages the fixed contact 21b. When the movable contact 23 dwells in the position 2311, the circuit of the motor 1 is open and the circuit of the electromagnet I1 is completed from one pole of the energy source 24, through the contacts 22, 22a of the selector switch D, contacts 20, 20a of limit switch A, the winding of the electromagnet 11, contact 23 of the master switch E, and to the other pole of the energy source.

If the operator thereupon desires to replace the slide whose image is being projected onto the screen with a fresh slide, the master switch E is actuated by moving the contact 23 from the position 23a to the position 23b. This opens the circuit of the electromagnet 11 so that the motion-transmitting element 7 of the power train 2 is not braked and the switch E completes the circuit of the motor I from the one pole of the energy source 24, by way of the contact 23 (in position 23b), the winding of the motor 1, and to the other pole of the energy source. The movable contact 23 can be released almost immediately so that it returns to the normal position 23a (e.g., under the action of a suitable spring, not shown) but the circuit of the motor 1 remains completed because the camshaft 6 turns the cam 18 so that the latter moves the contact 20 of the limit switch A against the contact 20b to complete the motor circuit from the one pole of the energy source 24, by way of the contacts 22, 22a of the selector switch D, contacts 20, 20b of the limit switch A, the winding of the motor I, and. to the other pole of the energy source. The circuit of the motor I is opened again when the camshaft 6 completes a full revolution, i.e., when the cam 18 again moves the contact 20 of the limit switch A into engagement with the fixed contact 200. At the same time, the limit switch A completes the circuit of the electromagnet 11 so that the latter's armature 12 engages the ring 8 and brakes the power train 2. The braking action can be readily selected in such a way that the camshaft 6 comes to a halt in an exactly determined position.

When the camshaft 6 dwells in the angular position of FIG. 2, the cam 19 causes the movable contact 21 of the limit switch B to engage the fixed contact 21b. If the operator thereupon desires to effect rotation of the camshaft 6 through 180, the selector switch D is actuated to move the contact 22 into engagement with the fixed contact 22b. The circuit of the motor 1 is completed immediately by way of the switches D, B and the motor begins to turn the camshaft 6 to change the position of the contact 20 in the limit switch A; this causes deenergization of the electromagnet 11. The motor 1 is. arrested when the camshaft 6 completes an angular movement through 180 because the contact 21 of the limit switch B then engages the contact 21a. This causes the limit switch B to open the motor circuit and to complete the circuit of the electromagnet l1, i.e., the armature 12 engages the ring 8 and the power train 2 is braked to come to a halt in an accurately determined angular position of the camshaft 6. The tray is then ready to be moved by hand, either for the purpose of withdrawing it from the housing of the slide projector or to move a selected slide into registry with the slide changer 16.

In order to thereupon return the slide changer 16 to the position in which a slide is located in the projection position, the operator returns the contact 22 of the selector switch D into engagement with the fixed contact 22a. Prior to such actuation of the selector switch D, the contact 20 of the limit switch A engages the contact 20b because the angular position of the camshaft 6 differs by 180 from that shown in FIG. 2. Therefore, the motor circuit is completed by way of the switches D, A so that the motor rotates the camshaft 6 and the circuit of the electromagnet 11 is open. The motor 1 is arrested when the camshaft 6 completes one-half of a revolution because the limit switch A then reassumes the position shown in FIG. 3 to open the motor circuit and to complete the circuit of the electromagnet 11. Thus, the camshaft 6 is braked again to assume a predetermined angular position in which the slide changer l6 maintains a slide in projection position.

It will be noted that the electromagnet 11 is mounted to brake an intermediate motion-transmitting element of the power train 2, namely, the gear 7 which is disposed between the first and last elements 4, 5 of the power train. Such positioning of the electromagnet 11 has been found to be advantageous because the electromagnet 11 need not produce a very strong braking force and because the electromagnet is capable of arresting the camshaft 6 in an accurately determined position. The power train can be arrested with the exertion of weakest braking force if such braking force is applied to the first element (pinion 4) and the camshaft 6 can be arrested with utmost accuracy if the braking force is applied to the last element (gear 5) of the power train 2. Thus, the positioning of the electromagnet 11 adjacent to the gear 7 is a compromise which insures stoppage of the camshaft 6 in a position which is at least close to the optimum position and with the exertion of a relatively weak braking force.

FIGS. 4 and 5 illustrate a modified driving mechanism wherein the power train includes a first gear 30 which is driven by the motor 1 (not shown), a pinion 31 which is coaxially secured to the gear 30, an intermediate gear which normally meshes with the pinion 31, and an elongated pinion 27 which is coaxially secured to the gear 25 and is in permanent mesh with the gear 5 on the camshaft 6. The manner in which the camshaft 6 cooperates with the limit switches A and B of the programming means (not shown in FIGS. 4 and 5) is the same as described in connection with FIGS. 2 and 3. The gear 25 is movable axially and is permanently biased to the position shown in FIG. 4 by a leaf spring 29 or a like biasing means. The electromagnet 111 is fixedly mounted in the housingof the slide projector and its stationary core 112 is separated from a soft iron ring 26 on the adjoining end face of the gear 25 by a gap 28 when the electromagnet 111 is deenergized. When the latter is energized, the core 1 l2 attracts the ring 26 and the gear 25 so that the gear 25 movesaway from mesh with the pinion 31 and the power train is interrupted. At the same time, the leaf spring 29 produces a stronger braking action because it is stressed in response to movement of the gear 25 from the position of FIG. 4 to the position of FIG. 5. Such braking action is assisted by frictional engagement between the core 112 and the ring 26 so that the camshaft 6 comes to a halt in an accurately determined position.

In this embodiment of my invention, the motor can continue to run itself out after the gear 25 is disengaged from the pinion 31 because such further rotation of the motor cannot influence the angular position of the camshaft 6. It is preferred to provide the gear 25 and the pinion 31 with suitably inclined teeth so that the gear 25 can reengage the pinion 31 in practically all angular positions of these parts. Such reengagement takes place in automatic response to deenergization of the electromagnet. 111 because the leaf spring 29 is then free to return the gear 25 to the axial position shown in FIG. 4. The parts 25, 31 constitute the members of a coupling which is disengaged in response to energization of the. electromagnet l1 1 with attendant interruption of the positive driving connection between the prime mover and the camshaft 6 and with the attendant automatic braking of the camshaft as well as of all those parts of the power train which are permanently connected with the camshaft. The gear 25 and its soft iron ring 26 constitute the armature of the electromagnet 111.

The driving mechanism of FIGS. 4 and 5 also insures that the camshaft 6v is arrested in accurately determined positions with the exertion of relatively small forces. This is attributed to the fact that the electromagnet 111 need not overcome the inertia of the entire power train because the motion-transmitting elements 30, 31 and the motor of the power train are automatically disconnected from the remaining elements of the power train when the electromagnet is energized. It will be noted that the parts 25, 27 constitute the next-to-the-last element of the power train (the last element is the gear 5) so that the application of braking force against such element can insure stoppage-of the camshaft 6 in accurately determined angular positions.

FIG. 6 illustrates a third embodiment of the invention wherein the power train between the motor (not shown) and the camshaft 6 comprises the gear 5, an elongated pinion 33 which is in permanent mesh with the gear 5, a gear 32which can be coupled with the pinion 33, a pinion 31 which meshes with the gear 32, and a gear which is driven by the motor. When the electromagnet 111 is deenergized, a leaf spring 38 biases the pinion 33 against the gear 32 so that the power train is operative and rotates the camshaft 6 as soon as the gear 30 begins to rotate in response to completion of the motor circuit. The gear 32 is provided with a friction lining 34 which is engaged by a similar friction lining 35 on the adjacent end face of the pinion 33 when the latter is free to follow the bias of the leaf spring 38. A soft iron ring 37 at the other end face of the pinion 33 is engaged and braked by the core 112 when the electromagnet 111 is energized whereby the resulting friction causes the camshaft 6 to come to a halt in a predetermined angular position. The pinion 33 is reciprocable and freely rotatable on a stub shaft 36 which is preferably secured to the gear 32. This pinion and its soft iron ring 37 constitute the axially movable armature of the electromagnet 111.

The soft iron rings 8, 26 and 37 are desirable when the gears of the power train consist of nonmetallic material. It is customary to employ in slide projectors gears which are made of synthetic plastic material. However, it is clear that such soft iron rings can be omitted if the material of the gear 7, 25 and/or 33 is a metal which is a good conductor ofmagnetic flux.

The braking action of the electromagnet 111 can, be enhanced by employing a yokelike core consisting of soft iron and having two arms or prongs disposedmirror symmetrically with reference to the axis of the braked motion transmitting element of the power train.

It is clear that the improved driving mechanism can be used with equal advantage in many other types of apparatus wherein one or more driven elements must be arrested in predetermined positions and wherein the stoppage should occur with a shot delay following inactivation of the primemover or disengagement of a coupling. For example, the driving mechanism can be used to effect axial adjustments of the projector lens.

It is further clear that the motor which drives the gear 30 of FIGS. 4-5 or FIG. 6 can run continuously because the fact thatthe gear 30 rotates after energization of the electromagnet 111 does not influence the braking action upon those elements of the power train which drive the camshaft 6.

It is also within the purview of my invention to employ an electromagnet as a means for directly braking the driven element which receives motion from the driving mechanism. For example, andreferring again to FIG. 2, the slide projector can be provided with two, electromagnets one of which can be energized to directly brake the slide changer 16 andthe other of which can be energized to directly brake the indexing mechanism 17 and/or the tray in response to opening of the circuit of the motor 1. The arrangement shown in FIGS. 1-3' is preferred at this time because 'a single electromagnet suffices to effect stoppage of one, two or more driven elements in accurately determined positions.

Without further analysis, the foregoing will. so fully reveal the gist-of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of my contribution to the art.

What I claim as new anddesired to be protected by Letters Patent is set forth in the appended claims:

1. A driving mechanism, particularly for effecting intermittent movements of parts. in slide projectors, comprising a prime. mover including an output member; a driven element; a power train for connecting said output member with said driven element; said power train comprising a plurality of motion transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements; resilient support means for said electromagnet means, said support means being operative to disengage said electromagnet means from said one element in deenergized condition of said electromagnet means and to store energy in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.

2. A-drivingmechanism as defined in claim 1, wherein said support means comprises a leaf spring.

3. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with' said driven element, said power train comprising a plurality ofmotion-transmitting elements; braking means including lectromagnet means energizable to thereby produce asbra ing action upon at least one of said elements, said electromagnet means comprising an armature which is moved into frictional engagement with said one element in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.

4. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a 4 prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements, said electromagnet means including a stationary portion and said one element constituting an armature which is moved into frictional engagement with said stationary portion in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.

5. A driving mechanism as defined in claim 4, further comprising biasing means for yieldably urging said armature away from engagement with said stationary portion.

6. A driving mechanism as defined in claim 4, wherein said one element is rotatable about a predetermined axis and is movable axially into and away from engagement with said stationary portion.

7. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements, said one element being rotatable about a predetermined axis and comprising a ring-shaped portion adjacent to said electromagnet means and consisting'of soft iron; and programming means for energizing said electromagnet means in predetermined positions of said driven element.

8. A driving mechanism as defined in claim 7, wherein said one element is a gear having an end face and wherein said ring is concentric with said gear and is adjacent to said end face thereof.

9. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train' for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements, said power train further comprising a coupling which is disengaged in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.

10. A driving mechanism as defined in claim 9, wherein said coupling includes a first coupling member driven by said prime mover and a second coupling member constituting said one element and normally in motion-receiving engagement with said first coupling member, said electromagnet means being arranged to disengage one of said coupling members from the other coupling member in response to energization thereof to thereby interrupt said power train.

11. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors comprising an electric motor including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements; and programming means for energizing said electromagnet means in predetermined positions of said driven element, said braking means comiarisirilgswitch means in circuit with said motor.

l A nving mechanism as defined in claim 11, wherein said switch means is operative to open the motor circuit in response to energization of said electromagnet means and to deenergize said electromagnet means in response to completion of the motor circuit.

13. A driving mechanism as defined in claim 12, wherein said switch means includes at least one limit switch and said driven element includes a rotary member having actuating means arranged to actuate said limit switch in at least one predetermined angular position of said rotary member.

14. A driving mechanism as defined in claim 13, wherein said switch means includes a plurality of two-way limit switches which change positions in response to successive angular displacements of said rotary member by 15. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one element of said power train; and pro gramming means for energizing said electromagnet means in predetermined positions of said driven element.

16. A driving mechanism as defined in claim 15, wherein said one element comprises a portion adjacent to said electromagnet means and consisting of soft iron. 

1. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element; said power train comprising a plurality of motion transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements; resilient support means for said electromagnet means, said support means being operative to disengage said electromagnet means from said one element in deenergized condition of said electromagnet means and to store energy in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.
 2. A driving mechanism as defined in claim 1, wherein said support means comprises a leaf spring.
 3. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements, said electromagnet means comprising an armature which is moved into frictional engagement with said one element in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.
 4. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements, said electromagnet means including a stationary portion and said one element constituting an armature which is moved into frictional engagement with said stationary portion in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.
 5. A driving mechanism as defined in claim 4, further comprising biasing means for yieldably urging said armature away from engagement with said stationary portion.
 6. A driving mechanism as defined in claim 4, wherein said one element is rotatable about a predEtermined axis and is movable axially into and away from engagement with said stationary portion.
 7. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements, said one element being rotatable about a predetermined axis and comprising a ring-shaped portion adjacent to said electromagnet means and consisting of soft iron; and programming means for energizing said electromagnet means in predetermined positions of said driven element.
 8. A driving mechanism as defined in claim 7, wherein said one element is a gear having an end face and wherein said ring is concentric with said gear and is adjacent to said end face thereof.
 9. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements, said power train further comprising a coupling which is disengaged in response to energization of said electromagnet means; and programming means for energizing said electromagnet means in predetermined positions of said driven element.
 10. A driving mechanism as defined in claim 9, wherein said coupling includes a first coupling member driven by said prime mover and a second coupling member constituting said one element and normally in motion-receiving engagement with said first coupling member, said electromagnet means being arranged to disengage one of said coupling members from the other coupling member in response to energization thereof to thereby interrupt said power train.
 11. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors comprising an electric motor including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at least one of said elements; and programming means for energizing said electromagnet means in predetermined positions of said driven element, said braking means comprising switch means in circuit with said motor.
 12. A driving mechanism as defined in claim 11, wherein said switch means is operative to open the motor circuit in response to energization of said electromagnet means and to deenergize said electromagnet means in response to completion of the motor circuit.
 13. A driving mechanism as defined in claim 12, wherein said switch means includes at least one limit switch and said driven element includes a rotary member having actuating means arranged to actuate said limit switch in at least one predetermined angular position of said rotary member.
 14. A driving mechanism as defined in claim 13, wherein said switch means includes a plurality of two-way limit switches which change positions in response to successive angular displacements of said rotary member by
 180. 15. A driving mechanism, particularly for effecting intermittent movements of parts in slide projectors, comprising a prime mover including an output member; a driven element; a power train for connecting said output member with said driven element, said power train comprising a plurality of motion-transmitting elements; braking means including electromagnet means energizable to thereby produce a braking action upon at leaSt one element of said power train; and programming means for energizing said electromagnet means in predetermined positions of said driven element.
 16. A driving mechanism as defined in claim 15, wherein said one element comprises a portion adjacent to said electromagnet means and consisting of soft iron. 